Urea synthesis with improved heat recovery and conversion

ABSTRACT

Heat recovery and conversion of reactants to urea are improved in adiabatic and in isothermal urea synthesis systems.

FIELD OF THE INVENTION

This invention relates to the synthesis of urea from NH₃ and CO₂. Morespecifically, it relates to a new method of removing heat from anexothermic urea synthesis reactor operating in the pressure range from130 to 400 Kg/cm², in the temperature range from 160° to 220° C, in anNH₃ to CO₂ overall reactor feed mol ratio from 2.7 to 7 to one and in aH₂ O to CO₂ overall reactor feed mol ratio from zero to 1.5 to one. Italso relates to a new method of increasing the conversion in thereactor.

BACKGROUND OF THE INVENTION

Urea is generally produced by the well known method of contacting NH₃and CO₂ to form ammonium carbamate and of dehydrating the latter tourea. The first reaction is instantaneous and substantially complete;the second one is much slower and incomplete, and it takes place only inthe liquid phase. It is also known that in the presence of excess NH₃the conversion of ammonium carbamate to urea is promoted, and that inthe presence of excess water it is hindered.

The formation of ammonium carbamate is strongly exothermic, and thedehydration of ammonium carbamate to urea is endothermic, but to alesser degree. For this reason, generally excess heat must be removedfrom the urea synthesis reactor if the formation of ammonium carbamateand the dehydration of carbamate to urea are simultaneously carried outin the same vessel.

The excess exothermic heat of reaction is usually removed from the ureasynthesis reactor by producing steam in a coil immersed in the ureasynthesis mixture. The common drawbacks of such a method of removal ofthe excess heat of reaction from a urea synthesis reactor are asfollows:

1. A RELATIVELY LARGE REACTOR COIL IS USUALLY REQUIRED DUE TO THE SMALLTEMPERATURE DIFFERENTIAL NORMALLY AVAILABLE BETWEEN THE SYNTHESISREACTOR MIXTURE AND THE BOILING CONDENSATE IN THE REACTOR COIL. Thisproblem becomes very much pronounced in the case that steam must beproduced in the reactor coil at a relatively high and usable level andat the same time the reactor is operated at a relatively high NH₃ to CO₂reactor feed mol ratio, for instance above about 3.4 to one. It is awell known fact that, in the presence of excess NH₃ in the ureasynthesis reaction mixture, the vapor pressure of the reaction mixtureis increased and its boiling point is decreased, thus requiring a lowerreactor operating temperature when operating at a predetermined andconstant reactor pressure level.

2. LOCAL OVERHEATING OF THE UREA SYNTHESIS REACTION MIXTURE OCCURS DUETO POOR HEAT TRANSFER RATE TO THE COIL, WITH CONSEQUENT VAPORIZING OFTHE REACTANTS NH₃ and CO₂, and consequent loss in conversion of ammoniumcarbamate to urea.

There are two specific cases in which the reactor coil usually is notrequired because of the fact that the excess exothermic heat availablein the urea synthesis reactor is absorbed by a relatively large amountof either excess NH₃ or carbamate recycle solution or both, fed to theurea synthesis reactor at a relatively lower temperature. For example,in the so called "ONCE-THROUGH Urea Synthesis Processes", theunconverted reactants present in the reactor effluent are not recycledback to the reactor, but they are separated as gas from the aqueous ureaproduct solution by steam heating at reduced pressure and are sent to anadjacent plant for recovery and for the production of either ammoniumsulfate or ammonium nitrate. In such Once Through Processes the amountof liquid NH₃ reactor feed can be increased in practice to the point atwhich all the excess exothermic heat available in the urea synthesisreactor is used internally for the heating of the excess liquid NH₃reactor feed stream to the reactor operating temperature inside thereactor. Customarily, in this case the liquid NH₃ reactor feed and thereactor operating temperature are maintained, respectively, at about 20°C. and about 180°-185° C.

Furthermore, in the so called "Partial or Total Carbamate SolutionRecycle Urea Synthesis Processes", the above-mentioned unconvertedreactants separated from the aqueous urea product solution in the formof a gaseous mixture, instead of being used for the production ofammonium sulfate or nitrate, are absorbed in water to form an ammoniacalaqueoous solution of ammonium carbamate, and are recycled into the ureasynthesis reactor partially or totally, at a usual temperature of about90°-100° C. In this latter case, the excess exothermic heat available inthe reactor is used internally to elevate the temperature of thecarbamate solution recycle stream from 90°-100° C. to theabove-mentioned reactor operating temperature of 180°-185° C. Obviouslyin such a case, if the amount of recycled carbamate solution isrelatively large, the corresponding amount of heat in deficiency must beadded to the reactor if one wishes to maintain its operating temperatureat a certain optimum and desired temperature level. This amount of heatin deficiency is usually added to the reactor by preheating therelatively cold liquid NH₃ reactor feed stream from the above-mentionedtemperatue of about 20° to 80° C. There is a common drawback to bothsuch specific cases with respect to the conversion in the reactor, aswill be explained below.

As discussed above, it becomes evident that in both cases, either in aOnce Through or in a Carbamate Solution Recycle urea synthesis process,the exothermic heat of reaction available in the urea synthesis reactoris wasted by being used internally for the sole purpose of bringing therelatively colder reactor feed streams up to the operating temperatureat which the urea synthesis reactor is maintained. Therefore, such areactor runs completely adiabatically, without heat removal or heataddition, once the reactor feed streams are introduced into the reactor.However, if the relatively colder reactor feed streams are preheatedbeyond the point at which a urea synthesis reactor operatesadiabatically, as for instance in accordance with the method describedin U.S. Pat. No. 3,579,636, it becomes necessary to remove from thereactor the equivalent amount of heat added to the reactor feed streamsin excess of the normal requirement of an adiabetic reactor. In such acase, the urea synthesis reactor becomes exothermic again, as forinstance in the case of the above mentioned Once Through urea synthesisprocesses.

As mentioned above, the problem of removing heat from the urea synthesisreactor becomes more complicated when excess NH₃ is used in the reactorfor the purpose of attaining a higher degree of conversion of ammoniumcarbamate to urea.

It has been found that by removing the excess heat of reaction,available in an exothermic reactor, in an external high pressure heatexchanger substantially operating at the same reactor pressure and at arelatively lower NH₃ to CO₂ molar ratio than the reactor, and that bysubsequently contacting the resulting reaction mixture with additionalexcess NH₃ in a substantially adiabatic urea synthesis reactor,considerable advantages are attained, as described further below.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a process wherein,the major part or all of the fresh make-up CO₂ gas required for theproduction of urea is mixed with NH₃ in an NH₃ to CO₂ molar ratio offrom about 2.2 to about 3.5 to 1, at reactor synthesis pressure, eitherin the shell side or in the tube side of a conventional shell and tubeheat exchanger. A stream containing one or more of the followingcompounds: water, ammonium carbamate, ammonia and urea can be admixedwith the above-mentioned fresh make-up CO₂ gas and NH₃ mixture. In sucha case the overall NH₃ to CO₂ molar ratio of the total resulting mixtureis maintained from about 2.2 to 1 to about 3.5 to 1. The temperature ofthe total reaction mixture inside the heat exchanger is maintainedconstant within a range from about 160° to about 220° C. by removingheat from the heat exchanger. Such heat is removed from the heatexchanger either by circulating a relatively colder fluid in indirectcontact with the reaction mixture, or by producing steam from condensatein indirect contact with the reaction mixture.

DRAWINGS

Advantages of the invention will become apparent to those skilled in theart from the following detailed description considered in conjunctionwith the drawings wherein

FIGS. 1 through 5, inclusive, are schematic flow diagrams of ureasynthesis reactor systems; and

FIG. 6 is a schematic diagram of a urea synthesis reactor having a heatexchange means therein.

SPECIFIC EMBODIMENTS OF THE INVENTION

The amount of heat removed from the above-mentioned heat exchanger iscontrolled by regulating the flow of NH₃ to the heat exchanger reactionmixture, and thus by regulating the overall NH₃ to CO₂ molar ratio inthe heat exchanger reaction mixture. According to this new method, inthe case of either removal of an excessive amount of heat from the heatexchanger (which fact will cause a drop in temperature of the subsequentadiabatic reactor as will be explained further below) or overheating ofthe reaction mixture, the feed rate of NH₃ admixed to the reactionmixture is increased. As a consequence, due to the increase in freeexcess NH₃ content in the reaction mixture, the vapor pressure of thereaction mixture will increase or the boiling point of the reactionmixture will decrease when operating at a constant pressure level, asmentioned above. As a further consequence, a smaller amount of heat willbe transferred to the relatively colder cooling medium circulatedthrough the heat exchanger due to the decrease in temperaturedifferential between the reaction mixture and the cooling mediumabsorbing heat. In the specific case in which the correct amount of heatis removed from the heat exchanger, but at the same time the temperatureof the reaction mixture is too high, an increase in NH₃ feed rate to thereaction mixture will cause the temperature of the reaction mixture todrop within the desired temperature range as explained above. In orderto restore the heat removal to the correct amount, it will be necessaryeither to reduce the temperature of the cooling medium absorbing heat orto increase its circulation rate, or to produce lower pressure steam inthe heat exchanger if heat is removed by boiling condensate. In eithercase the removal of the amount of heat will be restored to the correctvalue, but at a lower reaction mixture operating temperature, asdesired.

In the case of removing an insufficient amount of heat from the heatexchanger, which fact will cause an increase in temperature of thesubsequent adiabatic reactor as will be explained further below, thefeed rate of NH₃ admixed to the reaction mixture is decreased, for thepurpose of decreasing the free excess NH₃ content in the reactionmixture and thus of increasing the boiling point of the reactionmixture. Consequently, the temperature differential between the reactionmixture and the cooling medium absorbing heat will increase as well asthe rate of heat removal from the heat exchanger.

Once the required amount of heat is removed from the reaction mixture,the latter is reduced to the thermodynamic state and conditionsprevailing in and characteristic of the adiabatic reactor feed streams.The reaction mixture from the heat exchanger is then mixed withadditional NH₃ for the purpose of increasing the overall NH₃ to CO₂molar ratio in the total mixture to from about 2.8 to 1 to about 7 to 1,and is fed into an adiabatic reactor for conversion of ammoniumcarbamate to urea. The reactor temperature is controlled by removing therequired amount of heat from the preceding high pressure heat exchanger,very much analogously but in the opposite manner to the conventionalmethods of preheating the liquid NH₃ reactor feed stream of aconventional reactor for the purpose of controlling the temperature ofsuch a conventional adiabatic reactor, as was explained before.

In the process according to this invention, the pressure in the reactoris maintained at the substantially same pressure prevailing in theprocess side of the high pressure heat exchanger described above.

As mentioned above, there are still some problems that are common to theconventional exothermic or adiabatic reactors designed to operateaccording to conventional methods used heretofore. Normally, thereactants are fed into the bottom section of such a conventionaladiabatic reactor, and the synthesis mixture at completion of reactionis discharged from its top section. Such a conventional exothermic oradiabatic reactor usually consists of a cylindrical vertical vessel witha height-to-diameter ratio of from about 6 to about 20 to 1, for thepurpose of approaching as much as possible the ideal and in practicepreferred pattern of an upwardly plug type flow of the synthesis mixturethrough the reactor. But due to the facts mentioned above, according towhich the reaction of carbamate dehydration to urea is a slow reactionand is endothermic by about 7,000 Kcal/mole of urea formed, the bottomsection of such a conventional adiabatic reactor operates at atemperature which is from 10° to 15° C. higher than the temperature ofthe synthesis mixture at completion of the reaction at the exit from thereactor. In a few words, part of the exothermic and instantaneous heatof reaction of ammonium carbamate formation from NH₃ and CO₂ in thereactor bottom section of a conventional reactor is stored as sensibleheat in the synthesis mixture, with a consequent increase in temperatureof the synthesis mixture, to be later slowly released to supply theamount of endothermic heat required for the dehydration of ammoniumcarbamate to urea during the upward flow of the synthesis mixturethrough the reactor. Such a condition invariably leads in a conventionalreactor to undesirable local overheating of the synthesis mixture in thereactor bottom section, with consequent loss of NH₃ and CO₂ gaseousreactants from the liquid phase of the synthesis mixture, and withconsequent decrease in conversion of total CO₂ to urea. As a furtherdisadvantage of such a condition prevailing in a conventional reactor,the temperature of the synthesis mixture in the upper portion of thecylindrical reactor decreases in proportion to the amount of urea formedby the endothermic process of carbamate dehydration to urea and water.Due to this gradual drop in temperature of the synthesis mixture duringits upward flow through the reactor, the rate of carbamate dehydrationto urea is slowed down, with obvious negative effects on the overallconversion of total CO₂ present in the synthesis mixture to urea.

According to the new methods described herein, either the mixture of thereactor feed streams of the adiabatic reactor are counter-currentlycontacted in indirect heat exchange with the synthesis mixture; or thereactor feed streams are preheated below the point of attaining theadiabatic temperature equilibrium in the reactor at a certain preferredoperating temperature level and the reactor mixture is heated in thereactor; or heat is removed from the bottom section of the reactor andthe upper section of the reactor is heated at the same time. In thelatter case simultaneous cooling of the bottom section and heating ofthe upper section of the reactor can be accomplished practically byimmersing a coil into the reactor synthesis mixture, by vaporizingcondensate in the bottom part of the coil in contact with the relativelyhotter synthesis mixture and condensing the steam thus formed in theupper part of the coil in contact with the relatively cooler synthesismixture. By this method, excess heat from the bottom section of thereactor is transferred to the upper section of the reactor where it isrequired for the endothermic process of carbamate dehydration to urea.Such a reactor coil can be of the shape of an upper and lowercylindrical or spherical vessel connected with numerous tubes inparallel, wherein condensate is refluxing internally; more exactly it isvaporized in the bottom section and condensed in the upper section ofthe coil. This type of reactor is illustrated in FIG. 6 and is discussedhereinbelow.

In the second case mentioned above, either the reactor feed streams arepreheated below the point of attaining the adiabatic temperatureequilibrium in the reactor at a certain preferred and optimum operatingtemperature, or more heat is removed from the above-described highpressure reactor feed streams to the heat exchanger than is required tobring the reactor temperature down to the preferred optimum operatingtemperature. In either case, the equivalent amount of such heatsubtracted in excess from the reactor or insufficiently supplied to thereactor feed streams must be supplied to the reactor from an externalsource in order to maintain the reactor temperature at the samepreferred optimum level. This is accomplished by feeding steam to a coilimmersed into the synthesis mixture and extracting the condensate fromthe coil. The amount of heat added to such a coil is equivalent to fromabout 0.1 to about 12,000 preferably from about 2,000 to about 10,000,Kcal/Kg mole of urea formed in the reactor.

In the first case mentioned above, the simplest, method of preventingthe bottom section of an adiabatic reactor from overheating is to bringthe mixture of two or more reactor feed streams into indirectcounter-current heat exchange with the reactor synthesis mixture.However, the most ecomonmical and practical method of accomplishing saidindirect counter-current heat exchange is as follows. A part or thetotal amount, from about 5 to 100 mol percent, of one or more of thereactor feed streams, namely CO₂, NH₃ and an ammoniacal aqueous solutionof ammonium carbamate which may contain urea, is simultaneouslycontacted and introduced at the top section of the reactor into a coilwhich is immersed into the synthesis mixture and runs internally to thereactor, from its top section to its bottom section. The coil immersedinto the reactor synthesis mixture is open at its end in the bottomsection of the reactor. The mixture of the reactor feed streams in theirdownward path through the reactor coil releases parts of its heat ofreaction and exits from the coil into the bottom section of the reactor.The remaining portion of the reactor feed stream or streams notintroduced into the reactor coil is fed to the bottom part of thereactor and mixed with the mixture exiting from the reactor coil. Theresulting total mixture rises through the reactor counter-currently tothe fluid flowing downwardly inside the reactor coil, and as thedehydration of carbamate to urea progresses with consequent drop intemperature of the reactor synthesis mixture, heat is indirectlytransferred from the fluid flowing downwardly through the reactor coilto the urea synthesis mixture flowing upwardly through the reactor.

The reactor synthesis mixture at completion of carbamate dehydration tourea is withdrawn from the top section of the reactor for furtherprocessing, not described here.

The following examples serve to illustrate preferred embodiments of theinvention.

EXAMPLE 1

A conventional Once Through vertical cylindrical reactor provided withan internal steam coil, is operated at a temperature of about 188° C.and at a pressure of about 270 atmospheres.

4,400 Kg/Hr of CO₂ at about 10° C. and 6,800 Kg/Hr of NH₃ at about 16°C. are fed to the reactor, in an approximate NH₃ to CO₂ overall molarratio of 4 to 1 to yield about 4,200 Kg/Hr of urea, 2,300 Kg/Hr ofunconverted ammonium carbamate, 3,400 Kg/Hr of excess NH₃ and 1,260Kg/Hr of water. On this basis, the overall calculated conversion of CO₂to urea is in the order of about 70 percent.

In order to maintain the reactor temperature constant at about 188° C.,it is necessary to remove excess heat from the reactor. This isaccomplished by producing steam in a reactor coil immersed in thesynthesis mixture, at about 9 atmospheres absolute pressure and about176° C., in an amount of approximately 1,080 Kg/Hr.

The surface area of the reactor coil is approximately equivalent to 64m² and the temperature differential available between the reactorsynthesis mixture and the boiling condensate producing steam inside thereactor coil is about 11° C.

Referring to FIG. 1, 4,400 Kg/Hr of CO₂ at about 10° C. in pipe 1 and4,420 Kg/Hr of NH₃ at about 16° C. in pipe 2 are fed into the bottomsection 3 of the tube side of vertical shell and tube heat exchanger 4,operating at about 270 atm. pressure. The overall NH₃ to CO₂ molar ratioin the feed to heat exchanger 4 is about 2.6 to 1. About 1,000 Kg/Hr ofcondensate is passed through pipe 5 into the shell side 6 of heatexchanger 4 to produce steam at about 9 atmospheres absolute pressureand about 176° C., which is extracted through pipe 7. The steam pressurein shell side 6 is controlled by means of valve 8 located in steam exitpipe 7.

The temperature of the reaction mixture inside the tubes of heatexchanger 4 is at about 200° C., and thus the temperature differentialavailable between the reaction mixture contained in tubes 10 (not shown)and the boiling condensate contained in shell 6 is about 24° C. Thetotal amount of tube surface area required in heat exchanger 4 isequivalent to about 28 m², only about 45 percent of the tube surfacearea required for a conventional reactor coil. The reaction mixture frompipe 9 is passed into reactor coil 11, provided with an open end 12proximate the bottom section 13 of the reactor 14. The reaction mixturefrom pipe 9 flows downwardly through coil 11 and is cooled from about200° C. to about 180° C. The reaction mixture exits coil 11 at its openend 12 and it is mixed with 2,380 Kg/Hr of ammonia at about 16° C.passed through pipe 15 and introduced into the bottom section 13 ofreactor 14, operating at about 270 atmospheres pressure. The resultingsynthesis mixture is further cooled below 180° C. due to said mixing ofthe reaction mixture leaving coil 11 at its end 12 and the relativelycolder stream of ammonia in pipe 15. The synthesis mixture, now in a NH₃to CO₂ molar ratio of about 4 to 1, flows through the reactor upwardlyand in counter-current heat exchange with the relatively hotter reactionmixture flowing downwardly through reactor coil 11. The synthesismixture is heated to about 188° C. and, at completion of reaction ofcarbamate conversion to urea, is withdrawn from the upper section 16 ofreactor 14 through pipe 17.

The stream flowing through pipe 17 contains 4,560 Kg/Hr of urea, 1,872Kg/Hr of unconverted ammonium carbamate, 3,400 Kg/Hr of excess NH₃ and1,368 Kg/Hr of water. On this basis, the back calculated conversion oftotal CO₂ to urea is about 76 percent, compared to 70 percent attainedin a conventional reactor described above.

EXAMPLE 2

A conventional vertical cylindrical reactor is adiabatically operated ina total recycle urea synthesis process plant at a temperature of about190° C. and at a pressure of about 220 atmospheres.

3,124 Kg/Hr of CO₂ at about 120° C., 5,182 Kg/Hr of NH₃ at about 70° C.and 5,246 Kg/Hr of ammonium carbamate recycle solution at about 85° C.are fed to the above-mentioned conventional reactor. The carbamaterecycle solution contains 3,276 Kg/Hr of ammonium carbamate, 690 Kg/Hrof NH₃ and 1,280 Kg/Hr of water. No reactor cooling or heating isrequired, thus the reactor operates adiabatically. The overall NH₃ toCO₂ molar ratio in the reactor is about 3.8 to 1 and the water to CO₂molar ratio is about 0.629 to 1. At completion of reaction, the reactoreffluent stream contains 4,200 Kg/Hr of urea, 3,354 Kg/Hr of unconvertedammonium carbamate, 3,458 Kg/Hr of excess NH₃ and 2,540 Kg/Hr of water.On this basis, the back calculated conversion of total CO₂ to urea isabout 62 percent.

According to this invention and referring to FIG. 2, 3,124 Kg/Hr of CO₂at about 115° C. in line 20, 4,821 Kg/Hr of NH₃ at about 35° C. in line21 and 4,407 Kg/Hr of ammonium carbamate recycle solution at about 80°C. in line 22 are introduced into the bottom section 23 of verticalcylindrical reactor 24 internally provided with heating coil 25 immersedinto the reaction mixture therein. The carbamate solution in line 22comprises 2,496 Kg/Hr of ammonium carbamate, 745 Kg/Hr of NH₃ and 1,166Kg/Hr of water. Reactor 24 is operated at a pressure of about 220atmospheres pressure, the NH₃ to CO₂ overall molar ratio in reactor 24is about 3.8 to 1, and the water to CO₂ molar ratio therein is about0.629 to 1.

750 Kg/Hr of steam at 16 atmospheres absolute pressure is supplied toreactor heating coil 25 through line 26 for the purpose of maintainingthe reactor temperature constant at about 190° C. Condensate is removedfrom heating coil 25 through line 27. At completion of reaction thereactor effluent is removed from reactor 24 through line 28. Theeffluent contains 4,200 Kg/Hr of urea, 2,574 Kg/Hr of unconvertedammonium carbamate, 3,152 Kg/Hr of excess NH₃ and 2,426 Kg/Hr of water.On this basis, the overall back calculated conversion of total CO₂ tourea is about 68 percent compared with the relatively lower conversionof 62 percent attained in the conventional reactor described above,operating at the same NH₃ to CO₂ and H.sub. 2 O to CO₂ molar ratios.

EXAMPLE 3

A conventional vertical cylindrical reactor is adiabatically operated ina total recycle urea synthesis process plant at a temperature of about190° C. and at a pressure of about 220 atmospheres. 3,124 Kg/Hr of CO₂at about 120° C., 5,499 Kg/Hr of NH₃ at about 75° C., and 4,728 Kg/Hr ofammonium carbamate solution at about 85° C., are fed to the abovementioned conventional reactor. The carbamate recycle solution contains2,918 Kg/Hr of ammonium carbamate, 600 Kg/Hr of NH₃ and 1,210 Kg/Hr ofwater. No reactor cooling or heating is required, thus the reactor isoperated adibatically. The overall NH₃ to CO₂ molar ratio in the reactoris about 4.0 and 1 and the water to CO₂ molar ratio is about 0.62 to 1.At completion of reaction, the reactor effluent stream contains 4,200Kg/Hr of urea, 2,996 Kg/Hr of unconverted ammonium carbamate, 3,685Kg/Hr of excess NH₃ and 2,470 Kg/Hr of H₂ 0. On this basis, the backcalculated conversion of Total CO₂ to urea is about 64 percent.

According to this invention and referring to FIG. 3, 3,124 Kg/Hr of CO₂at about 115° C. in line 31, 2,729 Kg/Hr of NH₃ at about 46° C. in line32, and 4,063 Kg/Hr of ammonium carbamate solution at about 80° C. inline 33, are mixed and are introduced into reactor coil 34 insidereactor 35. The overall NH₃ to CO₂ molar ratio of the mixture in reactorcoil 34 is about 2.6 to 1. This mixture flows downwardly through reactorcoil 34, leaves reactor 34 at its open end 36 located proximate reactorbottom section 37. The mixture is mixed with 2,400 Kg/Hr of NH₃ at about46° C., which is added to the bottom section 37 of reactor 35 throughline 38. The total amount of NH₃ fed to the reactor in line 39 is thusequal to 5,129 Kg/Hr and the overall NH₃ to CO₂ molar ratio in the totalresulting reactor mixture in bottom section 37 is about 4.0 to 1,whereas the water to CO₂ molar ratio is about 0.62 to 1. Reactor 35 isoperated adiabatically at about 190° C. and about 220 atmospherespressure. No heating or cooling is required, and the reactor temperatureis maintained constant at about 190° C. by varying the temperature ofone or more of the reactor feed streams 31, 32, 33, 38 or 39,respectively. At completion of reaction, the reactor effluent is removedthrough overhead line 40. The effluent contains 4,200 Kg/Hr of urea,2,340 Kg/Hr of unconverted ammonium carbamate, 3,400 Kg/Hr of excess HN₃and 2,376 Kg/Hr of water. Based on this, the back calculated overallconversion of total CO₂ to urea is about 70 percent, compared to therelatively lower conversion of 64 percent attained in the conventionalreactor described above and operating at the same NH₃ to CO₂ and H₂ O toCO₂ molar ratios.

EXAMPLE 4

A conventional vertical cylindrical reactor is adiabatically operated ina total recycle urea synthesis process plant at a temperature of about188° C. and at a pressure of about 220 atmospheres. 3,124 Kg/Hr of CO₂at about 120° C, 5,928 Kg/Hr of NH₃ at about 72° C. and 4,510 Kg/Hr ofammonium carbamate recycle solution at about 88° C., are fed to theabove described conventional reactor. The carbamate recycle solutioncontains 2,730 Kg/Hr of ammonium carbamate, 630 Kg/Hr of NH₃ and 1,150Kg/Hr of water. No reactor cooling or heating is required. The overallNH₃ to CO₂ molar ratio in the reactor is about 4.3 to 1 and the water toCO₂ molar ratio is about 0.60 to 1. At completion of reaction, thereactor effluent stream contains 4,200 Kg/Hr of urea, 2,808 Kg/Hr ofunconverted ammonium carbamate, 4,144 Kg/Hr of excess NH₃ and 2,410Kg/Hr of water. On this basis, the calculated conversion of total CO₂ tourea is about 66 percent.

According to this invention and referring to FIG. 4, 3,124 Kg/Hr of CO₂at about 120° C. in line 41, 3,661 Kg/Hr of carbamate recycle solutionat about 120° C. in line 42, and 2,560 Kg/Hr of NH₃ at about 120° C. inline 43, are fed to the tube side 44 of vertical shell and tube heatexchanger 45. The carbamate recycle solution in line 42 contains 1,950Kg/Hr of ammonium carbamate, 670 Kg/Hr of NH₃ and 1,041 Kg/Hr of water.The heat of reaction is removed from heat exchanger 45 by producing1,870 Kg/Hr of steam at about 179° C. and about 9.8 atmospheres absolutepressure in shell side 46 of heat exchanger 45. Condensate is fed toshell side 46 through line 47 and the steam produced is removed throughline 48. The reactor mixture leaves heat exchanger 45 at about 200° C.through overhead line 49 in an overall NH₃ to CO₂ molar ratio of about2.5 to 1, and is introduced into bottom section 50 of reactor 51. 2,937Kg/Hr of NH₃ at about 120° C in line 52 is also introduced into bottomsection 50 and is mixed with the reaction mixture from line 49, so thatthe total overall NH₃ to CO₂ molar ratio of the resulting reactormixture in bottom section 50 is equal to about 4.3 to 1, whereas thewater to CO₂ molar ratio is about 0.60 to 1. Reactor 51 and tube side 44of heat exchanger 45 operate at substantially the same pressure of about220 atmospheres. Reactor 51 is provided with steam coil 53 immersed inthe synthesis mixture therein. The temperature inside reactor 51 ismaintained constant at about 190° C. by supplying 600 Kg/Hr of steam atabout 210° C., and about 18.8 atmospheres absolute pressure to heatingcoil 53 through line 54. Condensate is extracted from coil 53 throughline 55. At completion of reaction, the resulting reaction mixture isextracted from reactor 51 through line 56 located in the upper part ofreactor 51. The mixture in line 56 4,200 Kg/Hr of urea, 2,028 Kg/Hr ofunconverted ammonium carbamate, 3, 753 Kg/Hr of excess NH₃ and 2,301Kg/Hr of water. On this basis, the calculated conversion of total CO₂ tourea is about 73 percent, compared to 66 percent conversion attained inthe conventional reactor described above and operating at the same NH₃to CO₂ molar ratio of 4.3 to 1 and at the same H₂ O to CO₂ molar ratioof 0.60 to 1.

EXAMPLE 5

According to this invention and referring to FIG. 5, 3,124 Kg/Hr of CO₂at about 120° C. in line 61, 3,661 Kg/Hr of ammonium carbamate recyclesolution at about 120° C. in line 62, and 2,560 Kg/Hr of NH₃ at about120° C. in line 63, are fed to the tube side 64 of vertical shell andtube heat exchanger 65. The carbamate recycle solution in line 62contains 1,950 Kg/Hr of ammonium carbamte, 670 Kg/Hr of NH₃ and 1,041Kg/Hr of water. The heat of reaction derived from the mixing of saidreactor feed streams is removed from heat exchanger 65 by producing1,250 Kg/Hr of steam at about 179° C. and about 9.8 atmospheres absolutepressure in shell side 66 of heat exchanger 65. Condensate is fed toshell side 66 through line 67 and the steam produced is removed throughline 68.

The reaction mixture in tubes 64, at about 220 atmospheres pressure andabout 200° C., is removed from heat exchanger 65 through overhead line69. The reaction mixture has a NH₃ to CO₂ molar ratio of about 2.5 to 1.Stream 69 is introduced into reactor coil 70 at top section 71 ofreactor 72. Reactor coil 70 is immersed into the reactor synthesismixture and it extends from its top section 71 downwardly to its bottomsection 74. Reactor coil 70 is open at its end 73 in reactor bottomsection 74. The relatively hotter stream from line 69 flows downwardlythrough reactor coil 70 and counter-currently to the reactor synthesismixture which is rising through the reactor 72. The reaction mixtureleaving coil 70 at its bottom open end 73 is mixed in reactor bottomsection 74 with 2,937 Kg/Hr of NH₃ at about 120° C. from line 75, sothat the overall NH₃ to C0₂ molar ratio of the reactor synthesis mixturein bottom section 74 and in reactor 72 is about 4.3 to 1, and the H₂ Oto CO₂ molar ratio is about 0.60 to 1, similar to Example 4 above.

The reactor synthesis mixture in reactor bottom section 74 is cooledbelow 186° C due to mixing with the relatively colder NH₃ stream 75 fedto reactor 72, rises through reactor 72 and is heated by indirect heatexchange with the relatively hotter stream 69 flowing counter-currentlythrough reactor coil 70, as explained above. At completion of reaction,the resulting reactor synthesis mixture is withdrawn from reactor topsection 71 through overhead line 76 at about 220 atmospheres pressureand about 190° C. The mixture in line 76 contains 4,200 Kg/Hr of urea,2,028 Kg/Hr of unconverted ammonium carbamate, 3,753 Kg/Hr of excess NH₃and 2,301 Kg/Hr of water. On this basis, the conversion of total CO₂ tourea is about 73 percent, compared to 66 percent conversion attained ina conventional reactor described in Example 4 and operating at the sameNH₃ and H₂ O to CO₂ molar ratios, respectively.

FIG. 6 illustrates an alternative reactor which has feed lines 51, 52and 53 connected thereto and an output line 54 for removing the reactionmixture therefrom. Inside the reactor 50 is a heat exchanger 55 whichcomprises a condensate header 56 in the lower portion of the reactor andan upper header 57 in the upper portion of the reactor. Conduits 58interconnect the lower and upper headers. A heat exchange medium iscontained within the headers and conduits of the heat exchange means.Optionally, an input line 59 for supplying a heat exchange medium isprovided to the upper header 57 and an output line for removing heatexchange medium is provided from lower condensate header 56.

In the embodiment where input and output lines 59 and 60 are notprovided, the heat exchange means 55 comprises a closed systemconsisting of headers 56 and 57 and conduits 58 having a heat exchangemedium therein. Heat of reaction at the lower portion of the reactor istransferred to the heat exchange medium in header 56 and is preferablyconverted into latent heat. That is, in the event that the heat exchangemedium is steam, the heat of reaction at the lower portion of thereactor converts condensate into steam at the lower portion of the heatexchange means. The heated steam then passes upwardly through conduits58, as does the reaction product in the reactor. During the upward flow,heat from the steam is transferred to the reaction product and thesteam, after being spent, becomes condensate and passes through conduits58 back to condensate header 56 in the lower portion of the reactor. Inthis manner, the reaction product is heated at the upper portions of thereactor in an efficient and simple manner.

Optionally, the spent heat exchange medium can be removed from the upperheader 57 and re-circulated back to the condensate heater 56 by externalpiping. A still further alternative is to supply a heated heat exchangemedium to the upper header 57, the heat exchange medium being forceddownwardly through conduits 58 to condensate header 56 and then removedfrom the system via conduit 60. This is an opened type system, whereasthe preceding embodiments are closed type systems.

Thus, with the reactor of FIG. 6, a liquid heat exchange medium isvaporized in the bottom section of the heat exchanger located in a lowerportion of the reactor, by the heat of reaction generated by thereactants fed into the reactor via lines 51-53. As the heated heatexchange medium passes to an upper portion of the system, its heat istransferred to the rising and cooling reactants and is eventuallycondensed in the upper section of the reactor. The condensate may berefluxed either internally (via conduits 58) or externally, or may beremoved from the system and fresh heat medium supplied to the system.Steam and other conventional heat exchange media can be used.

I claim:
 1. In a substantially adiabatic urea synthesis system whereinfluid NH₃ and fluid CO₂ are reacted at elevated pressure and temperatureto form a total urea synthesis mixture in a reactor having a conduitposition therein, the improvement comprisingcharging reactor feedstreams comprising from about 5 to about 100 mol percent of each of areactor feed stream of fluid NH₃ and fluid CO₂ into an upper section ofsaid conduit and forming therein a first urea synthesis mixture, passingsaid first urea synthesis mixture downwardly through said conduit anddischarging it therefrom into said reactor proximate the reactor bottomsection. charging the balance of said fluid NH₃ and fluid CO₂ reactorfeed streams directly into said reactor bottom section to form a totalurea synthesis mixture therein, passing said total urea synthesismixture in said reactor indirectly and countercurrently to said firsturea synthesis mixture to maintain a substantially constant temperaturethroughout said reactor, and wherein said first urea synthesis mixtureand said total urea synthesis mixture are formed at substantially thesame elevated pressure.
 2. The process of claim 1, wherein the NH₃ toCO₂ molar ratio inside said conduit is maintained at from about 2.2 toabout 3.5 to 1, and additional NH₃ is fed to the bottom section of thereactor and is mixed therein with a stream discharged from said conduitproximate the reactor bottom section to provide a resulting mixturehaving a NH₃ to C0₂ molar ratio of from about 2.8 to about 7 to
 1. 3.The process of claim 1, wherein the NH₃ to CO₂ molar ratio inside saidconduit is maintained at from about 2.2 to about 3.5 to 1, andadditional CO₂ and NH₃ are fed to the bottom section of the reactor andare mixed therein with said first urea synthesis mixture discharged fromsaid conduit proximate the reactor bottom section to provide a resultingmixture having a NH₃ to CO₂ molar ratio of from about 2.8 to about 7to
 1. 4. The process of claim 1, wherein said first urea synthesismixture is formed from said fluid NH₃ and fluid CO₂ streams and areactor feed stream of an aqueous solution containing ammoniumcarbamate, and the total NH₃ to total CO₂ molar ratio inside the conduitis maintained at from about 2.2 to about 3.5 to
 1. 5. The process ofclaim 4, wherein the NH₃ to CO₂ ratio inside said conduit is maintainedat from about 2.2 to about 3.5 to 1, and additional NH₃ is fed to thebottom section of the reactor and is mixed therein with said first ureasynthesis mixture discharged from said conduit proximate the reactorbottom section to provide a resulting mixture having a NH₃ to CO₂ molarratio of from about 2.8 to about 7 to
 1. 6. The process of claim 4,wherein the NH₃, to CO₂ molar ratio inside said conduit is maintained atfrom about 2.2 to about 3.5 to 1, and additional CO₂ and NH₃ are fed tothe bottom section of the reactor and are mixed therein with said firsturea synthesis mixture discharged from said conduit proximate thereactor bottom section to provide a resulting mixture having a NH₃ toCO₂ molar ratio of from about 2.8 to about 7 to 1.